The system of the present invention is applicable to any crane of level-beam design, wherein the load is transported horizontally by moving a trolley out along a beam. Crane systems of this type include gantry cranes and overhead-transport devices; and the present invention is particularly adapted for, and is further described herein for, loading cranes used for loading container cargo onto ships at pierside. Exemplary systems suitable for practice of the present invention are shown in U.S. Pat. Nos. 5,089,972 and 5,142,658.
Despite efforts to improve and automate the process of loading containers onto ships at pierside, the mode of operation continues to be manually intensive and time consuming. The principal factor in the inefficiency of the operation is at the end of each loading operation ("move") when the operator attempts to pick up or place the load. Sway induced while the load (or empty spreader) was transported between pier and ship must be killed by the operator during the move itself or at the end. Only the most experienced operators can simultaneously kill sway and bring the load or spreader to a specified target location; the rest must accomplish these goals in two separate operations. As a result, the time spent waiting for the container to stop swinging and fine-positioning it at the end of the loading operation occupies, on average, more than one third of the entire move.
One way to reduce this waiting time is to employ an anti-sway trolley motion control law, that meets the operator's velocity or position demand, and yet produces zero net sway at the end of the move. To be successful, such a control law must be safe, and must be amenable to manual operation, wherein trolley velocity reference signals are generated by an operator's control stick and the control law is designed to meet this reference value with no residual sway. There is also a smoothness constraint, imposed by the fact that the operator co-controls the process, and may be physically located in a cab attached to the trolley. Finally, there are external causes of sway, such as wind, crane motion, and non-vertical lifting of the load, so an effective system should be able to remove sway induced by such external factors.
The primary sources of sway are the initial acceleration of the trolley in the direction of its destination, and the final deceleration to stop the trolley at the end of the move. The sway caused by initial acceleration is unavoidable if the load is to be moved at all. However, this sway can be efficiently and smoothly removed before the trolley reaches its reference velocity, by a previously known "double pulse" anti-sway control law whereby the sway induced by an initial acceleration is removed by a second acceleration of the same sign, magnitude, and duration, timed to commence one-half a sway period after the commencement of the first pulse. To meet a given velocity reference, the first acceleration pulse is of sufficient length to accelerate to one-half the reference velocity; the second acceleration pulse then accelerates the trolley to the full reference velocity. To stop the load, the reference velocity is simply set to zero, and the same double-pulse method is applied to decelerate to this new reference without residual sway. The basic double-pulse approach is taught by U.S. Pat. No. 4,756,432.
This process can be extended naturally in two previously-known ways. First, if the load suspension system is linear, the double pulse strategy can be used to meet arbitrary varying trolley velocity demands by superposing the response to each new velocity demand on the responses to the previous ones Second, if the hoist length changes, the change in pendulum period can be accommodated by normalizing the measurement of time in dividing by the sway period. If, for example, the first acceleration pulse is executed and then the load is raised, shortening the hoist length and hence the sway period, then the second, anti-sway acceleration pulse will occur earlier than for the longer hoist length, and will be of shorter duration. These extensions are taught by U.S. Pat. No. 3,517,830 and particularly by Virkkunen, U.S. Pat. No. 5,127,533.
In Virkkunen's system, variable control intervals are employed to handle the change in pendulum period when the load is hoisted or lowered. However, Virkkunen's system has the drawback, that when the hoist length is changed, the anti-sway acceleration pulses will no longer be of the same duration as the accelerations with which they are paired. Thus, if the hoist length is changed, the original acceleration meets half the trolley velocity reference, but the antisway accelerations will no longer integrate to the other half of the reference velocity because the antisway pulses are now of longer or shorter duration than the original pulses. Consequently, the system will exceed or fall short of the target velocity.
Further, in the prior art no account is made for the energy added to, or subtracted from, the pendulum as a result of hoisting while the load is swinging; and existing double-pulse controls do not account for externally-induced sway from wind, from initial sway, and the like.
Accordingly, while the Virkkunen law has been applied successfully to shop cranes with long hang lengths and low hoist rates, it is not adequate for container cranes, which have relatively short hang lengths and high hoist rates; which operate outdoors; and where speed demands (especially for zero speed) must be met precisely.
It is an object of the present invention to provide an alternative to the basic double pulse law that eliminates the drawbacks of existing systems referred to above; that is, to control the trolley accelerations in such a way that the reference velocity is met exactly, that hoist-induced sway is fully corrected, and that externally-induced sway can be removed.
Another object of the present invention is to provide a safe control system for minimizing sway in movement of containerized cargo by an overhead crane assembly.
An additional object of the present invention is an automated, anti-sway, system for cranes that can be co-controlled by the crane operator, can be overridden by the crane operator, and is also capable of being operated in the manual mode by the crane operator.
Another object of the present invention is an anti-sway system for crane operations that eliminates sway caused by external wind and ship motion in loading of containerized cargo onto ships.